A typical radio transceiver comprises an antenna, a signal processing unit for baseband processing of received signals and signals that are to be transmitted. Between the antenna and the signal processing unit are a receive chain and a transmit chain. The receive chain converts received radio frequency (RF) signals down to baseband for further processing by the signal processing unit. The transmit chain generally converts signals from baseband to RF for transmission from the antenna. The receive chain comprises an input amplifier which amplifies the received signal, a mixer which mixes the amplified signals with a signal from a local oscillator (LO) to convert to Intermediate frequency (IF), and another mixer which mixes the IF signal with a signal from another oscillator to convert to baseband. The transmit chain can comprise another mixer which mixes the baseband signals with a signal from another LO to upconvert to RF and an output amplifier which amplifies the RF signals for transmission.
The LOs in the transmit and receive chains could be constituted by a single oscillator. The LOs signals may be required to switch from one frequency to another in order to transmit or receive on different frequencies. This requires the LO to tune to the desired frequency in a rapid and accurate manner.
Furthermore, in many applications it would be desirable, in order to reduce size and cost, to implement the entire transceiver, on a single integrated circuit (IC). One particular difficulty in designing such an IC is the implementation of the LOs and the circuitry required to tune the LOs to the desired operational frequencies. Additionally, there is increasing market demand for lower power products.
There is therefore a need for an accurately tunable LO whilst utilising a small on-chip area and low power.